Turbine disks used in gas turbine engines employed to support rotating turbine blades encounter different operating conditions radially from the center or hub portion to the exterior or rim portion. The turbine blades are exposed to high temperature combustion gases which rotate the turbine. The turbine blades transfer heat to the exterior portion of the disk. As a result, these temperatures are higher than those in the hub or bore portion. The stress conditions also vary across the face of the disk. Until recently, it has been possible to design single alloy disks capable of satisfying the varying stress and temperature conditions across the disk. However, increased engine efficiency in modern gas turbines as well as requirements for improved engine performance now dictate that these engines operate at higher temperatures. As a result, the turbine disks in these advanced engines are exposed to higher temperatures than in previous engines, placing greater demands upon the alloys used in disk applications. The temperatures at the exterior or rim portion may be 1500.degree. F. or higher, while the temperatures at the bore or hub portion will typically be lower, e.g., of the order of 1000.degree. F.
In addition to this temperature gradient across the disk, there is also a variation in stress, with higher stresses occurring in the lower temperature hub region, while lower stresses occur in the high temperature rim region in disks of uniform thickness. These differences in operating conditions across a disk result in different mechanical property requirements in the different disk portions. In order to achieve the maximum operating conditions in an advanced turbine engine, it is desirable to utilize a disk alloy having high temperature creep and stress rupture resistance as well as high temperature hold time fatigue crack growth resistance in the rim portion and high tensile strength, and low cycle fatigue crack growth resistance in the hub portion.
Current design methodologies for turbine disks typically use fatigue properties, as well as conventional tensile, creep and stress rupture properties for sizing and life analysis. In many instances, the most suitable means of quantifying fatigue behavior for these analyses is through the determination of crack growth rates as described by linear elastic fracture mechanics ("LEFM"). Under LEFM, the rate of fatigue crack propagation per cycle (da/dN) is a function which may be affected by temperature and which can be described by the stress intensity range, .DELTA.K, defined as K.sub.max.sup.-K min. .DELTA.K serves as a scale factor to define the magnitude of the stress field at a crack tip and is given in general form as .DELTA.K=f(stress, crack length, geometry).
Complicating the fatigue analysis methodologies mentioned above is the imposition of a tensile hold in the temperature range of the rim of an advanced disk. During a typical engine mission, the turbine disk is subject to conditions of relatively frequent changes in rotor speed, combinations of cruise and rotor speed changes, and large segments of cruise component. During cruise conditions, the stresses are relatively constant resulting in what will be termed a "hold time" cycle. In the rim portion of an advanced turbine disk, the hold time cycle may occur at high temperatures where environment, creep and fatigue can combine in a synergistic fashion to promote rapid advance of a crack from an existing flaw. Resistance to crack growth under these conditions, therefore, is a critical property in a material selected for application in the rim portion of an advanced turbine disk.
For improved disks, it has become desirable to develop and use materials which exhibit slow, stable crack growth rates, along with high tensile, creep and stress-rupture strengths. The development of new nickel-base superalloy materials which offer simultaneously the improvements in and an appropriate balance of tensile, creep, stress-rupture, and fatigue crack growth resistance, essential for advancement in the aircraft gas turbine art, presents a sizeable challenge. The challenge results from the competition between desirable microstructures, strengthening mechanisms, and composition features. The following are typical examples of such competition: (1) a fine grain size, for example, a grain size smaller than about ASTM 10, is typically desirable for improving tensile strength but not creep/stress-rupture and crack growth resistance; (2) small shearable precipitates are desirable for improving fatigue crack growth resistance under certain conditions, while shear resistant precipitates are desirable for high tensile strength; (3) high precipitate-matrix coherency strain is typically desirable for good stability, creep-rupture resistance and probably good fatigue crack growth resistance; (4) generous amounts of refractory elements such as W, Ta or Nb can significantly improve strength, but must be used in moderate amounts to avoid unattractive increases in alloy density and to avoid alloy instability; (5) in comparison to an alloy having a low volume fraction of the ordered gamma prime phase, an alloy having a high volume fraction of the ordered gamma prime phase generally has increased creep/rupture strength and hold time resistance, but also increased risk of quench cracking and limited low temperature tensile strength.
Once compositions exhibiting attractive mechanical properties have been identified in laboratory scale investigations, there is also a considerable challenge in successfully transferring this technology to large full-scale production hardware, for example, turbine disks of diameters up to, but not limited to, 25 inches. These problems are well known in the metallurgical arts.
A major problem associated with full-scale processing of Ni-base superalloy turbine disks is that of cracking during rapid quench from the solution temperature. This is most often referred to as quench cracking. The rapid cool from the solution temperature is required to obtain the strength required in disk applications, especially in the bore region. The bore region of a disk, however, is also the region most prone to quench cracking because of its increased thickness and thermal stresses compared to the rim region. It is desirable that an alloy for turbine disk applications in a dual alloy turbine disk be resistant to quench cracking.
Many of the current superalloys intended for use as disks in gas turbine engines operating at lower temperatures have been developed to achieve a satisfactory combination of high resistance to fatigue crack propagation, strength, creep and stress rupture life at these temperatures. An example of such a superalloy is found in the commonly-assigned application Ser. No. 06/907,276 filed Sept. 15, 1986. While such a superalloy is acceptable for rotor disks operating at lower temperatures and having less demanding operating conditions than .those of advanced engines, a superalloy for use in the hub portion of a rotor disk at the higher operating temperatures and stress levels of advanced gas turbines desirably should have a lower density and a microstructure having different grain boundary phases as well as improved grain size uniformity. Such a superalloy should also be capable of being joined to a superalloy which can withstand the severe conditions experienced in the rim portion of a dual alloy disk of a gas turbine engine operating at lower temperatures and higher stresses. It is also desirable that a complete rotor disk in an engine operating at lower temperatures and/or stresses be manufactured from such a superalloy.
As used herein, yield strength ("Y.S.") is the 0.2% offset yield strength corresponding to the stress required to produce a plastic strain of 0.2% in a tensile specimen that is tested in accordance with ASTM specifications E8 ("Standard Methods of Tension Testing of Metallic Materials," Annual Book of ASTM Standards, Vol. 03.01, pp. 130-150, 1984) or equivalent method and E21. The term ksi represents a unit of stress equal to 1,000 pounds per square inch.
The term "balance essentially nickel" is used to include, in addition to nickel in the balance of the alloy, small amounts of impurities and incidental elements, which in character and/or amount do not adversely affect the advantageous aspects of the alloy.